The present invention relates to a method of forming a composite coating film on car bodies and so on and a composite coating film obtained by the method.
From environmental considerations, efforts have been made in recent years to develop water-based coating systems for reducing the levels of organic solvents in coatings. In automotive coating where it is common practice to apply a base coating and, then, a clear coating, attempts have been made to apply a water-based system to the base coating.
As a water-based base coat composition suited to automotive use, Japanese Kokai Publication Sho-63-193968 discloses a water-based coating composition comprising a water-dispersible film-forming acrylic emulsion polymer, a hydrophobic melamine resin and a pigment and Japanese Kokai Publication Sho-63-265974 discloses a water-based paint comprising a resin emulsion having a core component comprised of a C4-12 alkyl-containing monomer and a shell component containing an acrylamide or methacrylic acid.
However, with any of the water-based base coat compositions heretofore available generally, inclusive of those mentioned above, it has been found difficult to attain both coat appearance and workability, particularly workability under highly humid conditions.
The object of the present invention is to provide a method of forming a composite coating film which makes it possible to attain both appearance and workability, particularly attain workability, in a high-humidity environment.
The method of forming a composite coating film according to the present invention comprises applying a water-based base coat composition on a substrate, and then applying a clear coat thereonto,
wherein said water-based base coat composition comprises a pigment and a resin emulsion obtained by emulsion polymerization of an xcex1,xcex2-ethylenically unsaturated monomer mixture,
said xcex1,xcex2-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and containing at least 65% by weight of a (meth)acrylic ester whose ester moiety contains 1 or 2 carbon atoms.
It is to be understood that the hydroxyl value of said xcex1,xcex2-ethylenically unsaturated monomer mixture may be within the range of 10 to 150.
Furthermore, the emulsion polymerization may be carried out in two stages.
In addition, said pigment may be a luster pigment.
Furthermore, the water-based base coat composition may comprise a curing agent and/or a water-soluble acrylic resin.
The water-soluble acrylic resin may for example be obtainable by solution polymerization of a component containing not less than 65 weight % of a (meth)acrylic ester whose ester moiety contains 1 or 2 carbon atoms.
The composite coating film of the invention is obtainable by the above method of forming a composite coating film.
The method of forming a composite coating film according to the invention comprises applying a water-based base coat composition to a substrate and, then applying a clear coat composition thereon, in which said water-based base coat composition comprises a pigment and a resin emulsion obtained by emulsion polymerization of an xcex1,xcex2-ethylenically unsaturated monomer mixture, said xcex1,xcex2-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and containing at least 65% by weight of a (meth)acrylic ester whose ester moiety contains 1 or 2 carbon atoms.
The water-based base coat composition for use in the method of forming a composite coating film in accordance with the invention contains a pigment and a resin emulsion obtained by emulsion polymerization of an xcex1,xcex2-ethylenically unsaturated monomer mixture.
The amount of the (meth)acrylate ester whose ester moiety contains 1 or 2 carbon atoms as contained in the xcex1,xcex2-ethylenically unsaturated monomer mixture is not less than 65% by weight. If the proportion is less than 65%, the coating film obtained will be poor in appearance. The (meth)acrylate ester whose ester moiety contains 1 or 2 carbon atoms includes methyl (meth)acrylate and ethyl (meth)acrylate. In the context of the present invention, xe2x80x9c(meth)acrylic esterxe2x80x9d includes, within the meaning thereof, both of xe2x80x9cacrylic ester and methacrylic esterxe2x80x9d.
The xcex1,xcex2-ethylenically unsaturated monomer mixture has an acid value of 3 to 50, preferably 7 to 40. Any acid value less than 3 will not contribute to improved workability and an acid value exceeding 50 will lead to decreased water resistance of the coating film. When, on the other hand, the water-based base coat composition is required to have curability, the xcex1, xcex2-ethylenically unsaturated monomer mixture should have a hydroxyl value of 10 to 150, preferably 20 to 100. A hydroxyl value less than 10 will fail to provide sufficient curability while a hydroxyl value exceeding 150 will lead to decreased water resistance of the coating film. From the viewpoint of physical properties of the coating film, it is preferred that the polymer obtained by copolymerization of the above xcex1,xcex2-ethylenically unsaturated monomer mixture have a glass transition temperature between xe2x88x9220xc2x0 C. and 80xc2x0 C.
The xcex1,xcex2-ethylenically unsaturated monomer mixture can have such an acid value and/or hydroxyl value as mentioned above by containing an acid group- or hydroxyl group-containing xcex1, xcex2-ethylenically unsaturated monomer or monomers therein.
As the acid group-containing xcex1,xcex2-ethylenically unsaturated monomer, there may be mentioned acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalate, 2-acryloyloxyethyl succinate, xcfx89-carboxypolycaprolactone mono(meth)acrylate, isocrotonic acid, xcex1-hydroxy-xcfx89-((1-oxo-2-propenyl)oxy)poly(oxy(1-oxo-1,6-hexanediyl)), maleic acid, fumaric acid, itaconic acid, 3-vinylsalicylic acid, 3-vinylacetylsalicylic acid, 2-acryloyloxyethyl acid phosphate, 2-acrylamido-2-methylpropanesulfonic acid and the like. Among these, acrylic acid, methacrylic acid and acrylic acid dimer are preferred.
As the hydroxyl group-containing xcex1,xcex2-ethylenically unsaturated monomer, there may be mentioned hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methacryl alcohol, hydroxyethyl (meth)acrylate-xcex5-caprolactone adducts and the like. Preferred among these are hydroxyethyl (meth)acrylate, hydroxybutyl (meth)acrylate, and hydroxyethyl (meth)acrylate-xcex5-caprolactone adducts.
The xcex1,xcex2-ethylenically unsaturated monomer mixture may contain one or more other xcex1,xcex2-ethylenically unsaturated monomers. Such other xcex1,xcex2-ethylenically unsaturated monomers include (meth)acrylate esters whose ester moiety contains 3 or more carbon atoms (e.g. n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, tert-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, dihydrodicyclopentadienyl (meth)acrylate, etc.), polymerizable amide compounds (e.g. (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-dibutyl(meth)acrylamide, N,N-dioctyl(meth)acrylamide, N-monobutyl(meth)acrylamide, N-monooctyl(meth)acrylamide, 2,4-dihydroxy-4xe2x80x2-vinylbenzophenone, N-(2-hydroxyethyl)acrylamide, N-(2-hydroxyethyl)methacrylamide, etc.), polymerizable aromatic compounds (e.g. styrene, xcex1-methylstyrene, vinyl ketones, tert-butylstyrene, p-chlorostyrene, vinylnaphthalene, etc.), polymerizable nitriles (e.g. acrylonitrile, methacrylonitrile, etc.), xcex1-olefins (e.g. ethylene, propylene, etc.), vinyl esters (e.g. vinyl acetate, vinyl propionate, etc.), dienes (e.g. butadiene, isoprene, etc.), and so forth. One or more of these monomers may be selected according to the intended application. For assuring hydrophilicity with ease, the use of (meth)acrylamide is preferred.
The proportion of said xcex1,xcex2-ethylenically unsaturated monomer or monomers other than the (meth)acrylate ester whose ester moiety contains 1 or 2 carbon atoms should be less than 35% by weight in the xcex1,xcex2-ethylenically unsaturated monomer mixture.
The resin emulsion to be contained in the water-based base coat composition of the invention is a resin obtained by emulsion polymerization of the above xcex1,xcex2-ethylenically unsaturated monomer mixture. Here, the emulsion polymerization can be carried out by a method generally well known in the art. Specifically, it can be effected by dissolving an emulsifier in water or an aqueous medium containing an organic solvent such as an alcohol, as necessary, and adding dropwise the above xcex1,xcex2-ethylenically unsaturated monomer mixture and a polymerization initiator thereinto with heating and stirring. It is also possible to add dropwise the xcex1,xcex2-ethylenically unsaturated monomer mixture emulsified beforehand using an emulsifier and water in the same manner.
Suited for use as the polymerization initiator are oil-soluble azo compounds (e.g. azobisisobutyronitrile, 2,2xe2x80x2-azobis(2-methylbutyronitrile), 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile), etc.), water-soluble azo compounds (e.g. anionic 4,4xe2x80x2-azobis(4-cyanovaleric acid) and 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine and cationic 2,2xe2x80x2-azobis(2-methylpropionamidine)); as well as oil-soluble redox system peroxides (e.g. benzoyl peroxide, p-chlorobenzoyl peroxide, lauroyl peroxide, tert-butyl perbenzoate, etc.) and water-soluble redox system peroxides (e.g. potassium persulfate, ammonium persulfate, etc.).
The emulsifier may be any of those commonly used in the art. Particularly preferred are, however, reactive emulsifiers such as Antox MS-60 (product of Nippon Nyukazai), Eleminol JS-2 (product of Sanyo Chemical Industries), Adeka Reasoap NE-20 (product of Asahi Denka Kogyo), Aqualon HS-10 (product of Dai-ichi Kogyo Seiyaku) and the like.
A chain transfer agent such as a mercaptan (e.g. lauryl mercaptan) and xcex1-methylstyrene dimer may be used as necessary for controlling the molecular weight.
The reaction temperature depends on an initiator. Thus, with an azo initiator, for instance, it is 60 to 90xc2x0 C. and, with a redox system, the reaction is preferably carried out at 30 to 70xc2x0 C. Generally, the reaction time is 1 to 8 hours. The amount of the initiator relative to the total amount of the xcex1,xcex2-ethylenically unsaturated monomer mixture is generally 0.1 to 5% by weight, preferably 0.2 to 2% by weight.
The above emulsion polymerization may be carried out in a plurality of stages, for example in 2 stages. Thus, a portion of the xcex1,xcex2-ethylenically unsaturated monomer mixture (xcex1,xcex2-ethylenically unsaturated monomer mixture 1) is first subjected to emulsion polymerization and then the remaining xcex1,xcex2-ethylenically unsaturated monomer mixture (xcex1,xcex2-ethylenically unsaturated monomer mixture 2) is further reacted to conduct the emulsion polymerization.
From the standpoint of preventing the effect of imbibing on the clear coat, the xcex1,xcex2-ethylenically unsaturated monomer mixture 1 preferably contains an amide group-containing xcex1, xcex2-ethylenically unsaturated monomer. In that case, it is more preferable for the xcex1,xcex2-ethylenically unsaturated monomer mixture 2 to be free of any amide group-containing xcex1,xcex2-ethylenically unsaturated monomer. Since the xcex1,xcex2-ethylenically unsaturated monomer mixtures 1 and 2 combinedly constitute said xcex1,xcex2-ethylenically unsaturated monomer mixture, the above-mentioned requirements imposed on the xcex1,xcex2-ethylenically unsaturated monomer mixture are to be satisfied by the xcex1,xcex2-ethylenically unsaturated monomer mixtures 1 and 2 combined.
The thus-obtained resin emulsion preferably has a particle diameter within the range of 0.01 to 1.0 xcexcm. When the particle diameter is smaller than 0.01 xcexcm, the workability improving effect will not be appreciable. A particle diameter larger than 1.0 xcexcm may possibly impair the appearance of the resulting coating film. The particle size can be controlled by adjusting the monomer composition and/or emulsion polymerization conditions.
The above resin emulsion may be adjusted to pH 5xcx9c10 by neutralizing with a base, as necessary, and put to use. This is because the stability of the resin is high in this pH range. The neutralization is preferably carried out before or after emulsion polymerization by adding a tertiary amine such as dimethylethanolamine and triethylamine.
The pigment for use in the water-based base coat composition to be used in the practice of the invention includes a luster pigment and a color pigment. The luster pigment is not particularly restricted in form and shape. It may be colored. Preferably, however, it has a mean particle diameter (D50) of 2 to 50 xcexcm and a thickness of 0.1 to 5 xcexcm. A luster pigment having a mean diameter of 10 to 35 xcexcm is excellent in the quality of luster, hence is more preferred. Specifically, it includes colored or uncolored metal lusters, for example metals or their alloys such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide and the like, as well as mixtures thereof. It further includes interfering mica pigments, white mica pigments, graphite pigments and the like.
As the color pigment, on the other hand, there may be mentioned, among others, organic pigments such as azo chelate pigments, water-insoluble azo pigments, condensed azo pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments and metal complex pigments and inorganic pigments such as chrome yellow, yellow iron oxide, iron oxide red, carbon black and titanium dioxide.
The total pigment weight concentration (PWC) in the water-based base coat composition is preferably 0.1 to 50%, still more preferably 0.5 to 40%, most preferably 1.0 to 30%. Above the upper limit, the coating film obtained will be poor in appearance. In cases where a luster pigment is contained, it is generally preferred that the pigment weight concentration (PWC) thereof be not more than 18.0%. Above the upper limit, the coating film appearance will become poor. More preferably, that PWC is 0.01 to 15.0%, most preferably 0.01 to 13.0%.
The water-based base coat composition of the invention may contain a curing agent. The curing agent may be any of the curing agents used in coating compositions in general. As such, there may be mentioned amino resins, blocked isocyanates, epoxy compounds, aziridine compounds, carbodiimide compounds, oxazoline compounds and metal ions, among others. In consideration of coating film characteristics and production cost, amino resins and/or blocked isocyanates are generally used.
The amino resin as a curing agent is not particularly restricted but may be a water-soluble melamine resin or a water-insoluble melamine resin. From the viewpoint of the stability, a melamine resin having a water tolerance of not less than 3.0 is preferably used among melamine resins. The xe2x80x9cwater tolerancexe2x80x9d is a parameter used to evaluate the degree of hydrophilicity and a higher value of this parameter means a higher level of hydrophilicity. The value of water tolerance is determined by dispersing 0.5 g of a sample into 10 ml of acetone in a 100-ml beaker at 25xc2x0 C., adding deionized water gradually to this dispersion using a biuret and determining the amount (ml) of deionized water required to cause the mixture to become turbid. This amount (ml) of deionized water is referred to as the water tolerance value.
As the blocked isocyanate, there may be mentioned those products obtained by adding an active hydrogen-containing blocking agent to a polyisocyanate such as trimethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate and isophorone diisocyanate and capable of regenerating isocyanato groups upon dissociation of the blocking agent.
When such a curing agent is incorporated, its content is preferably 20 to 100 parts by weight per 100 parts by weight of the resin solid in the water-based base coat composition. Outside the above range, insufficient curability will result.
The water-based base coat composition of the invention may contain one or more other coating film-forming resins as necessary. Such resins are not particularly restricted but use may be made of acrylic resins, polyester resins, alkyd resins, epoxy resins, urethane resins and other coating film-forming resins.
The number average molecular weight range of said other coating film-forming resin or resins is 3,000 to 50,000, preferably 6,000 to 30,000. When the molecular weight is less than 3,000, coating workability is poor and curability is insufficient and when the molecular weight exceeds 50,000, the nonvolatile fraction is so low as to decrease the workability.
The other coating film-forming resin or resins preferably have an acid value of 10 to 100 mg KOH/g, more preferably 20 to 80 mg KOH/g. Above the upper limit, the water resistance of the coating film will decrease. Below the lower limit, the dispersibility of the resin in water will be decreased. Further, the resins preferably have hydroxyl values within the range of 20 to 180 mg KOH/g, more preferably 30 to 160 mg KOH/g. Above the upper limit, the water resistance of the coating film will be decreased while, below the lower limit, the curability of coating will be decreased.
The proportions of the resin emulsion and other coating film-forming resin in the water-based base coat composition are such that the resin emulsion accounts for 5 to 95% by weight, preferably 10 to 85% by weight, more preferably 20 to 70% by weight, and the other coating film-forming resin or resins account for 95 to 5% by weight, preferably 90 to 15% by weight, more preferably 80 to 30% by weight, based on the total resin solids. When the resin emulsion accounts for less than 5% by weight, workability tends to be adversely affected. When its proportion exceeds 95% by weight, the coating film-forming properties tend to become poor.
From the standpoint of compatibility with the resin emulsion, said other coating film-forming resin is preferably a water-soluble acrylic resin. The water-soluble acrylic resin can be obtained by solution-polymerizing an xcex1,xcex2-ethylenically unsaturated monomer having an acidic group as defined for said xcex1,xcex2-ethylenically unsaturated monomer mixture as an essential component together with one or more other xcex1,xcex2-ethylenically unsaturated monomers. From the standpoint of coating film appearance, it is preferable that the material to be used for said water-soluble acrylic resin contain not less than 65 weight % of a (meth)acrylic ester whose ester moiety contains 1 or 2 carbon atoms.
The water-soluble acrylic resin mentioned above is usually neutralized with a basic compound, for example an organic amine such as monomethylamine, dimethylamine, trimethylamine, triethylamine, diisopropylamine, monoethanolamine, diethanolamine and dimethylethanolamine and dissolved in water before use. The neutralization procedure may be applied to the very water-soluble acrylic resin or carried out in the stage of preparation of the water-based base coat composition to be described hereinafter.
The water-based base coat composition mentioned above may contain a polyether polyol. Incorporation of a polyether polyol contributes to the flip-flop properties of the composite coating film and, hence, improves the appearance of the coating film.
The polyether polyol mentioned above is preferably a compound containing at least one primary hydroxyl group per molecule and having a number average molecular weight of 300 to 3000, a hydroxyl value of 30 to 700, and a water tolerance value, defined above, of not less than 2.0. If the above requirements are not satisfied, the water resistance tends to be low and/or the expected improvement in appearance may not be obtained.
As such a polyether polyol, there can be used the adduct of an alkylene oxide, such as ethylene oxide and propylene oxide, to an active hydrogen-containing compound such as a polyhydric alcohol, a polyphenol compound and a polycarboxylic acid. As specific examples, such commercial products as Primepol PX-1000, Sannix SP-750 (both available from Sanyo Chemical Industries) and PTMG-650 (Mitsubishi Chemical), among others, can be mentioned. The amount of use of the polyether polyol in the coating is preferably 1 to 40 weight %, more preferably 3 to 30 weight %, on a solid matter basis.
The water-based base coat composition may contain an additional viscosity modifier to prevent imbibing with the top coat coating film and/or secure good workability in application. The viscosity modifier may be any of those generally showing thixotropic properties. As the viscosity modifier, there may be mentioned, for example, crosslinked or noncrosslinked resin particles, swollen dispersions of fatty acid amide, amide type fatty acids, polyamides such as long-chain polyaminoamide phosphates, polyethylene type ones such as colloidal swollen dispersions of oxidized polyethylene, organic acid smectite clays, montmorillonite and like organic bentonite type ones, inorganic pigments such as aluminum silicate and barium sulfate, and flat pigments capable of generating viscosity owing to the shape thereof.
The water-based base coat coating of the invention may contain, in addition to the above components, one or more of those additives generally used in coating compositions, such as surface modifiers, thickening agents, antioxidants, ultraviolet absorbers, antifoams, and so on. The addition amount of these are within the ranges respectively well known in the art.
The method of preparing the coating composition in the practice of the invention is not particularly restricted but may be any of all the methods known in the art, inclusive of those to be described later herein, for example the method comprising dispersing and kneading the pigment and other components using a kneader or roll mill.
The clear coat composition is not particularly restricted but may be any of clear coatings containing a coating film-forming resin, a curing agent and the like. It may contain a color pigment in an amount which will not impair the design of the undercoat. This clear coating may have a solvent-based or water-based form or a powder form.
As preferred examples of the solvent-based clear coating, there may be mentioned, from the viewpoint of transparency or acid etching resistance, for instance, combinations of an acrylic resin and/or polyester resin and an amino resin and/or isocyanate, or acrylic resins and/or polyester resins having a carboxylic acid-epoxy curing system.
As examples of the water-based clear coating, there may be mentioned a composition containing the coating film-forming resin mentioned hereinabove for formulation in the solvent-based clear coatings as rendered water-based type in advance by neutralization with a base. This neutralization can be carried out before or after polymerization by adding a tertiary amine such as dimethylethanolamine and triethylamine.
As for the powder-form clear coating, ordinary powder coatings such as thermoplastic or thermosetting powder coatings can be used. Thermosetting powder coatings are preferred, however, since they give coating films having good physical properties. Specific examples of the thermosetting powder coating are epoxy, acrylic and polyester type powder clear coatings, among which acrylic powder clear coatings capable of providing good weathering resistance are particularly preferred.
Further, to the above clear coating, there is preferably added a viscosity modifier to thereby secure good workability in application. The viscosity modifier may be any of those generally showing thixotropic properties. Usable as such are those mentioned hereinabove referring to the water-based base coat composition. The composition may contain a curing agent, a surface modifier and/or the like, where necessary.
The method of forming a coating film according to the invention can advantageously be applied to various substrates such as metals, plastics and foamed bodies and, in particular, to metal surfaces and castings. It can most judiciously be applied to metal products coatable by cationic electrodeposition coating.
As the metal products, there may be mentioned products made of iron, copper, aluminum, tin, zinc and the like as well as alloys containing these metals. Specifically, mention may be made of bodies and parts of cars, trucks, motorcycles, buses and the like. Most preferably, these metals are subjected in advance to chemical conversion treatment with a phosphate or chromate salt or the like.
An electrodeposition coating film may be formed on such a chemically converted steel panel surface and, in that case, the electrodeposition coating may be a cationic or anionic type coating. A cationic electrodeposition coating is preferred, however, since it gives a composite coating film superior in corrosion resistance.
As the plastics products, there may be mentioned products made of polypropylene resins, polycarbonate resins, urethane resins, polyester resins, polystyrene resins, ABS resins, vinyl chloride resins, polyamide resins and the like. Specifically, there may be mentioned spoilers, bumpers, mirror covers, grilles, door knobs and other automotive parts, among others. These plastics products are preferably washed by vapor cleaning using trichloroethane or washed with a neutral detergent. Further, they may be coated with a primer for enabling electrostatic coating.
When necessary, an intermediate coating film may be formed on the substrate. For forming such an intermediate coating film, an intermediate coating is used. The intermediate coating contains a coating film-forming resin, a curing agent, one or more of various organic or inorganic color pigments and extender pigments, and so forth.
The coating film-forming resin to be used in said intermediate coating is not particularly restricted but may be any of such coating film-forming resins as acrylic resins, polyester resins, alkyd resins, epoxy resins and urethane resins. These resins are used in combination with the curing agent described hereinbefore with reference to the water-based base coat composition. From the standpoint of film properties and cost, an amino resin and/or an isocyanate compound are/is generally employed.
As the color pigment to be contained in the intermediate coating, those pigments mentioned hereinabove referring to the water-based base coat composition can be used in the same manner. Normally, a gray intermediate coating in which carbon black and titanium dioxide are used as main pigments, a set gray composition matched in hue to the topcoat, or the so-called color intermediate coating in which various color pigments are used combinedly is preferred. Further, a flat pigment such as an aluminum or mica powder may be added.
In these intermediate coatings, there may be incorporated, in addition to the components mentioned above, one or more of those additives which are generally used in coating compositions, such as surface modifiers, antioxidants, antifoams, and so on.
According to the method of the invention for forming a composite coating film, a base coat and a clear coat can be formed, using the water-based base coat composition and the clear coat composition, respectively, in that order on a substrate, with an electrodeposited coating film and an intermediate coating film formed thereon as necessary.
In applying the water-based base coat composition according to the invention to car bodies, coating films can be formed by multistage coating, preferably two-stage coating, using the electrostatic air spray coating technique or by the coating method combinedly using an electrostatic air spray coater and a rotary atomizer type electrostatic coater commonly known as xe2x80x9cxcexcxcexc (micromicro) bellxe2x80x9d, xe2x80x9cxcexc (micro) bellxe2x80x9d or xe2x80x9cmetallic bellxe2x80x9d, for instance.
The coating film thickness to be attained in applying the water-based base coat composition in the practice of the invention may vary according to the intended application but a thickness of 10 to 30 xcexcm is useful in many instances. Above the upper limit, the image sharpness may decrease or troubles such as unevenness or runs may occur in the step of application. Below the lower limit, the substrate cannot be masked effectively.
While, in carrying out the method of forming a composite coating film according to the invention, it is also possible to apply a clear coat composition to the above base coat film after baking thereof, it is preferable from the economic and environmental viewpoint to apply the clear coat composition onto the uncured base coat to thereby form a clear coat, since, by doing so, a drying for baking can be omitted. For obtaining good finished coating films, it is desirable that the uncured base coat is heated at 40 to 100xc2x0 C. for 2 to 10 minutes prior to application of the clear coat composition.
In practicing the method of forming a coating film according to the invention, the clear coat applied after formation of the base coat is formed for the purpose of smoothing out the unevenness, twinklings and like effects caused by the base coat and protecting the same. As for the specific method of application, it is preferable to form a coat by using a rotary atomizer type electrostatic coater such as the xcexcxcexc bell or xcexc bell mentioned above.
As for the dry film thickness of the clear coat film formed by the clear coat composition, a thickness of about 10 to 80 xcexcm is generally preferred and a thickness of about 20 to 60 xcexcm is more preferred. Above the upper limit, troubles such as foaming or sagging may occur in the step of coating. Below the lower limit, the unevenness of the substrate cannot be masked.
Formation of the clear coat to be obtained in the above manner is preferably carried out by the so-called two-coat one-bake method, as mentioned above, namely followed by baking the same together with the uncured base coat. The baking temperature is selected from the range of 80 to 180xc2x0 C., preferably 120 to 160xc2x0 C., whereby a cured film with a high crosslinking density can be obtained. Above the upper limit, the coat may become hard and brittle and, below the lower limit, a sufficient level of hardness cannot be obtained. The curing time may vary depending on the curing temperature but, at 120xc2x0 C. to 160xc2x0 C., a time of 10 to 30 minutes is adequate.
The composite coating film formed according to the invention have, in many instances, a thickness of 30 to 300 xcexcm, preferably 50 to 250 xcexcm. Above the upper limit, the physical properties, such as thermal shock resistance, of the film is decreased while, below the lower limit, the strength of the coating film itself is sacrificed.
The method of forming a composite coating film according to the present invention in which a resin emulsion obtained from an xcex1,xcex2-ethylenically unsaturated monomer mixture of a defined formulation is used in the water-based base coat composition, it is possible to attain both good appearance and high coating workability at the same time. This is probably because the use of a high-SP neutral monomer in an amount not less than a given level results in an increased hydrophilicity of the resin without compromise in water resistance and other qualities, thus contributing to compatibility of the resin with water and, hence, an improved appearance of the coating film. On the other hand, because the resin has an acid value, the resin swells owing to the basic component present in the water-based coating so that a viscosity suited to an efficient coating workability is obtained. Thus, the method of forming a composite coating film insures good workability even under high-humidity coating conditions. In addition, a further improvement in film appearance can be obtained by incorporating a polyether polyol in the water-based base coat composition.